Development of solar cells or photovoltaic (PV) devices based on organic materials has become an important research field with substantial application perspectives. Features of these devices such as versatility and compatibility with flexible substrates combined with low-cost and large-area production provide them with advantages compared to conventional inorganic silicon-based solar cells. Among various organic semiconductors, liquid crystals (LCs) forming 2D columnar structures (see, C. Destrade, P. Foucher, H. Gasparroux, N. H. Tinh, A. M. Levelut, J. Malthete, Mol. Cryst. Liq. Cryst. 1984, 106, 121) are promising materials for PV applications (see, L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, E. Moons, R. H. Friend, J. D. Mackenzie, Science 2001, 293, 1119). In spite of much effort, the realization of efficient organic solar cells remains a major scientific challenge. There is a known photovoltaic converter based on poly(2-methoxy-5-(2′-ethyl-hexyioxy)-p-phenylenevinylene) (MEH-PPV) copolymer and a perylene-phenyl-ethyl-imide derivative (PPEI) (see J. J. Dittmer et al., Synthetic Metals, Vol. 102,879-880 (1999)). In this system, MEH-PPV acts as a hole acceptor, and PPEI acts as an electron acceptor (hole donor). Excitons which are photogenerated in the organic semiconductor subsequently decay into free charge carriers (electrons and holes) at the interface between the donor and acceptor components. Introduction of PPEI significantly increases the external quantum efficiency of photovoltaic devices employing this system. The PPEI particles are distributed in the MEH-PPV matrix volume over a distance equal to the exciton diffusion lenth (˜9 nm). Thus, a charge separation is stimulated in thin-film MEH-PPV structures in presence of PPEI.
Another known photovoltaic cell comprises the first layer of an organic electron donor material in contact with the second layer made of an organic electron acceptor material (Klaus Petritsch, PhD Thesis, “Organic Solar Cell Architectures”, Cambridge and Graz, July 2000. Chapter 4, Double Layer Devices, p. 67). These electron donor and electron acceptor materials are capable of absorbing light in a wavelength range from 350 to 1000 nm, and these materials are capable of forming a rectifying junction when contact with each other. The cell is provided with electrodes forming Ohmic contacts at least with a part of the surface of organic layers. A distinctive feature of said photovoltaic cell is that the organic materials employed contain organic compounds with generally planar polycyclic nuclei. These compounds are capable of forming a layer structure of a total thickness not exceeding 0.5 micron.
Organic layers of the above referenced materials do not possess crystalline structure which is a disadvantage for the photovoltaic devices. For this reason the mobility of electrons and holes in these layers is much lower as compared to that in the same bulk crystalline materials. As a result, electrons and holes do not leave the active region of a semiconductor structure during the exciton lifetime and recombine. Such electron-hole pairs do not contribute to the photocurrent, and the photovoltaic conversion efficiency decreases. In addition, a decrease in the electron and hole mobility leads to an increase in the resistivity of the material and, hence, in the serial resistance of the photovoltaic device. This implies increase of Ohmic losses and additional decrease in the photovoltaic conversion efficiency. Another disadvantage of the aforementioned photovoltaic devices is that the non-crystalline materials possess extremely small diffusion length of photogenerated excitons. This requires using photovoltaic structures of very thin layers of thickness comparable with the exciton diffusion length, and which also decreases both external and internal quantum efficiency of the photovoltaic devices.
The disclosed organic compounds and photovoltaic devices on their base are intended to overcome the disadvantages of the organic compounds and photovoltaic devices of the prior art.
Definitions of various terms used in the description and claims of the present invention are listed below.
The term “space group C2v” is illustrated in FIG. 1 and indicates that a molecule or molecular part has a 2-fold rotation axis AA′ and a mirror plane M1 parallel to the axis of rotation. Combination of the 2-fold axis and mirror plane gives one more mirror plate M2 which is parallel to the rotation axis and perpendicular to the first mirror plane M1.
The term “space group D2h” is illustrated in FIG. 2 and indicates that a molecule or molecular part has two 2-fold rotation axes AA′ and BB′ perpendicular to each other and mirror plane M1 perpendicular to one of them. This combination of the symmetry elements gives two additional mirror planes M2 and M3 perpendicular to each other and perpendicular to the initial mirror plane. It also provides one additional 2-fold, axis CC′ which is perpendicular to the initial 2-fold axes. An inversion center O is in the center of the molecular structure.